A last-minute prenup is among the worst mistakes a couple can make, says Robert Nachshin, a family-law attorney in Los Angeles. In California, prenups aren’t enforceable if there aren’t at least seven days between their being presented to the parties and the actual signing.

Mr. Nachshin suggests preparing the documents and having them reviewed and signed over a much longer window of time, at least six months before the wedding. “Prenup negotiation is very stressful. I’ve seen marriages break up because of prenup negotiations at the last minute,” he says.

Both individuals should have their own lawyers to oversee the negotiations and review the document, says Danielle Mayoras, a lawyer in Troy, Mich., and author of “Trial & Heirs, Famous Fortune Fights.”

“You cannot use the same lawyer and have one person protect both people,” Ms. Mayoras says.

Full financial disclosure from both sides is also important to putting together a solid prenup. “If you don’t know the other party’s assets, you cannot fully agree to the prenup because you don’t have all of the information that you need to make an educated decision,” Ms. Mayoras says. “The law always looks for a fair, arm’s-length transaction.”